This invention relates to ground leakage differential protective apparatus and, more particularly, to such apparatus which is substantially insensitive to the presence of accidental grounds in the neutral conductor between the apparatus and the load to be protected.
Currently, one of the most active areas in the field of electrical protection lies in differential ground leakage protection devices. The function of these devices, which are currently under development, is to disconnect the power supply very rapidly whenever a stray path to ground occurs. For example, if any animal or person should touch the energized, or hot, line leading to an electrical load, the presence of the extraneous circuit to ground will immediately trip a circuit breaker, deenergizing the load and thus saving the object of the ground from possible serious injury or death. Most of these devices operate on the differential transformer principle. A differential transformer customarily comprises an integral magnetic core in the form of a closed loop such as a toroid. Upon this core are positioned two primary windings of one or more turns which are connected in series with the respective line or neutral conductors feeding the load. These windings are so arranged that, when equal currents are passing through them, they produce opposing magnetic fluxes. A secondary winding is also mounted upon the core and is arranged to trip a circuit breaker in the line conductor. Under normal operation, all the current passing to the load through the line conductor returns to the supply through the neutral conductor. Thus, the respective magnetic fluxes cancel each other and no voltage is induced in the secondary winding. In the event of an accidental grounding of the line conductor, however, a portion of the line current will return to the power supply through the external ground. The resultant imbalance in the currents of the two primary windings creates a resultant flux which induces a voltage in the secondary winding and trips the circuit breaker.
Apparatus of the above type have proved to be very satisfactory under most circumstances. However, there is one disadvantage in apparatus of this type which could create serious safety problems. The problems with the prior apparatus arise if a low impedance ground occurs on the neutral conductor between the differential transformer and the load. If, concurrently with this condition, the line conductor were to become grounded, as through a person, the breaker would not operate at the same value of differential current. As one end of the primary winding in the neutral conductor is grounded at the power supply, a low impedance ground at its other end would, effectively, cause it to be short-circuited. Thus, there would be no potential difference existing across this particular winding. Electrically, the differential transformer would now resemble a current transformer wherein the winding in the line conductor would be the "primary" and the winding in the neutral conductor would be the shorted "secondary." The low impedance existing across this "secondary" winding would be reflected, in accordance with the turns ratio (normally 1:1), to the primary. Consequently, there would be no potential difference across the primary winding and it would be incapable of generating sufficient power in the transformer core to actuate the relay. Therefore, a greater amount of differential current would be required to achieve tripping. The lower the impedance to ground from the neutral conductor, the greater would be the differential current required to effect tripping. It will be immediately apparent to those skilled in the art that, under the proper conditions, this could create a serious safety hazard.